"Draw me a ball."
Translation:Gambarkan saya bola.
It makes it clear that there is only one ball. Sebuah + object = one object. For example, Saya membeli sebuah apel - I am buying an apple.
The same concept applies to seorang = one person, and seekor = one animal.
Dia diajar oleh seorang guru - She was taught by a teacher
Aku merawat seekor kucing - I care for a cat